A Vertical Cavity Surface Emitting Laser (VCSEL) is a laser resonator that includes two mirrors that are typically distributed Bragg reflector (DBR) mirrors which have layers with interfaces oriented substantially parallel to the die or wafer surface with an active region. The active region may include one or more bulk layers, quantum wells, quantum wires, or quantum dots for the laser light generation in between. The planar DBR-mirrors comprise layers with alternating high and low refractive indices. Each layer has a thickness of approximately one quarter of the laser wavelength in the material, or an odd integer multiple of the quarter wavelength, or in some cases even integer multiple of the quarter thickness, depending on layer placement and optical interference effects. The mirror layers can produce intensity reflectivities that may be above 99%, and in other cases may be produce much lower values of reflectivity. Slightly lower values than 99% can be useful to obtain high extraction efficiency of the laser light from VCSELs, and much lower values can be useful for RCLEDs or LEDs. Mirrors can also be made of other materials, including dielectrics or metals.
High-speed optical data networks, optical sensors, illuminating systems, and other optical systems can use VCSELs. Oxide-confined VCSELs were first demonstrated in 1993 and are commonly used in transmitters and transceivers for high-speed optical data networks. Oxide-confined VCSELs can be made to operate at speeds of 25 Gbps to 28 Gbps while retaining temperature performance needed for commercial applications.
RCLED's are described in U.S. Pat. No. 5,226,053. A RCLED is a light emitting diode (LED) that generates mainly spontaneous emission and generally operates without a distinct threshold. The drive voltage of a spontaneous emitter can be less than its photon energy divided by the electron charge, under which condition it ideally absorbs heat in its light emission process. The RCLED's drive voltage can also exceed its photon energy, under which it generally generates heat in its light emission.
Oxide VCSELs and oxide RCLEDs use an oxide aperture to obtain low threshold and high efficiency by providing high electrical current confinement and optical mode confinement to the transverse optical cavity. This aperture establishes the transverse modes of the lateral cavity. However, because of the oxide aperture, oxide VCSELs also suffer manufacturing and reliability problems caused by the oxide. Achieving electrical and mode confinement similar to oxide VCSELs without using the oxide aperture however has presented a hurdle to developing high performance vertical-cavity devices.
Selective oxidation has been used in attempts to fabricate other types of heterostructure devices, such as edge-emitting laser diodes, transistors or waveguides. However these devices suffer to a greater degree from material problems especially with the heterointerfaces that are also caused by the presence of the oxide layer. Therefore, despite the attractive properties of the selective oxidation, it has mainly been relegated in commercial devices to its wide-spread use in vertical cavity devices.
There are also other ways by which to make vertical cavity devices to create internal electrical confinement and optical confinement. For example, implantation process based on proton implantation is used to create proton-implanted VCSELs by implanting into high-energy protons to damage the crystal and cause a resistive aperture in the upper p-type mirror of a VCSEL. Other ion implanted impurity species have also been used to cause resistive regions through the crystal damage caused by the implant. These types of resistive regions caused by implant damage will generally regain unwanted conductivity if process steps that follow the implant are performed at too high of temperature. This also limits flexibility in VCSEL design. Another problem is that the implant regions cause excessive damage to the crystal that generally use thick implanted layers with heavy dose implants that result in large vertical and lateral straggle. Therefore the implanted regions should be kept away from the cavity spacer that causes high electrical resistance and low efficiency. Therefore both the thickness of the implanted region and its inability to retain high resistivity under high temperature process steps has led to generally unfavorable performance from proton implanted vertical-cavity devices.
Other semiconductor devices such as other types of semiconductor lasers, light emitting diodes, photodetectors and electronic devices all generally make use of current blocking regions. These devices can also benefit from improved forming of these current blocking regions.